There is a great need for high resolution thin film transistor/liquid crystal displays (TFTLCD) in a wide range of possible applications ranging from portable computers and test equipment to high resolution projection TV. Such displays typically consist of a large number of picture elements (pixel) arranged in an active matrix. For a display system where the electro-optical medium is liquid crystal, each pixel is defined by a dedicated electrode on one side of two opposite transparent substrates and another electrode, which is common to all pixels and faces the viewer.
FIG. 1 illustrates, in the form of an equivalent circuit, a prior art TFTLCD device. Each pixel in the active matrix comprises a TFT switch and a liquid crystal (LC) capacitor. The TFT gate electrodes in all the cells of a row are connected to a common horizontal gate bus, while the TFT sources in all the cells of each column are connected to a vertical data bus. The cells are addressed in a "line-at-a-time" or line-by-line mode. The timing waveforms for the display are also shown in FIG. 1. By pulsing a gate bus to a positive potential relative to the source potential during the addressing interval for a particular row, the TFTs in that row are all switched on. At the same time, the data signal voltages on the source busses are transferred to the TFT output electrodes (drains) and the LC capacitors. When the gate bus is switched off as the next row is addressed, the data signals are stored on the capacitors until the next addressing cycle for that particular row in the succeeding frame.
In order to obtain a full color display, a color filter mosaic of red, green and blue filter elements is placed on other substrates which are parallel to and spaced from the active matrix glass substrate to transmit lights of different colors. The elements of the filters are registered with the pixels so that each pixel is dedicated to one of the three primary colors. Group of justaposed red, green and blue pixels constitute color triplets whose primary color outputs combine to provide a multi-color display capability. By driving the array of pixels with appropriate red, green and blue data signals, a full color picture is produced.
It can be noted that with the above-described display system the number of row and column conductors needed corresponds to the number of rows n and columns m for n.times.m pixels. In addition to the need to devote a portion of area of the display device to accommodate the row and column conductors, there is also a possibility that, in view of the large number of conductors used, one or more of these conductors may be defective, rendering the display device unusable. This problem is quite common at the cross-overs of row and column conductors. Obviously, the more conductors are employed, the greater this possibility becomes to adversely affect to yield of a large area display device.
Furthermore, the large number of row and column conductors cause problems with the production of small area display devices which are used for projection display. Large area displays can be obtained from small area TFTLCD by using a projection system in which the image produced by the small area display is projected onto a large area screen. However, in order to provide the desired display resolution after projection, the display device generating the image should have adequate number of rows and columns of pixel density. If the number of the row and column conductors is large, a large portion of the display area is occupied by the conductors and the aperture ratio (i.e., display area where light can transmit/total area) on the display is small. Then the display exhibits low light levels.
An active matrix display system which can reduce the number of conductors is proposed in the U.S. Pat. No. 4,931,787, entitled "Active matrix addressed display system" by John M. Shannon. This patent proposed a TFTLCD system in which all the TFTs in the active matrix are n-channel amorphous TFTs arranged in groups. Each group contains a number of juxtaposed pixels sharing the same column and row conductors, as shown in FIG. 2(a). The TFTs of each group are fabricated to have different threshold voltages, i.e. TFT1, TFT2 and TFT3 have threshold voltages Vt1, Vt2 and Vt3 respectively. For Vt1 higher than Vt2 and Vt2 higher than Vt3, FIG.2(b) shows a part of a typical waveform for the switching signal Vg applied to a single row conductor. FIG.2(c) shows an example of video information waveform Vs for a single column waveform. At the beginning of a selected row energization period, the switching signal is at a high level, and all the TFTs, i.e. TFT1, TFT2 and TFT3, are switched on as this high voltage level exceeds all their threshold voltages. Simultaneously, the information signal S1 is applied to the associated column conductor, and is transferred to the drain electrodes of the TFTs so that all three pixel electrodes are charged to S1. At a certain time thereafter, the signal drops to an intermediate voltage level. At this time interval, TFT1 is turned off, and TFT2 and TFT3 are still on. Simultaneously with this intermediate switching signal level, the information signal applied to the column is changed to S2, and the pixel electrodes in contact with TFT2 and TFT3 are charged to S2. Then, after another predetermined period, the signals drops to a low level before finally dropping to zero, at which time both TFT1 and TFT2 are turned off and TFT3 is still on and the information signal is changed to S3. The pixel connected to TFT3 is then charged to S3.
Unfortunately, it is difficult to fabricate TFTs with different threshold voltages in the same process. Althrough using very complicated manufacturing process or connecting different capacitors in series with gate electrodes of different TFTs can make the TFTs to have different threshold voltages (i.e. Vt1, Vt2 and Vt3 for TFT1, TFT2 and TFT3 respectively), the threshold voltages cannot be controlled uniformaly within the whole large display area. If the threshold voltage of a TFT has a lower threshold voltage than its designed value, the leakage current makes the pixel not to hold the information signal during the subsequent period. That is, the electrode contact to TFT1 is charged to S2 if the threshold voltage of TFT1 is lower than its designed value. Then, the pixels address the wrong signal.
Another disadvantage of Shannon's method is the low noise margin of the switching signal. The TFTs are n-channel TFTs and the threshold voltage differences are small. As a result, the display has low switching signal noise margins.